<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .div1{
            background: red;
            width: 100px;
            height: 100px;
            transition: 1s;
        }
        .div1:hover{
            width: 200px;

        }
    </style>
</head>
<body>
<div class="div1"></div>
</body>
</html>